This invention relates generally to geophones, and more particularly to single-coil force-balance geophones.
Geophones are used in seismic imaging to measure velocity or acceleration. Commonly, a geophone includes a housing within which one or more coils are mounted within a magnetic field. The coils generally are mounted to the housing by way of a spring. Thus, movement of the housing results in movement of the coil within the magnetic field, generating an electromotive force (emf) in the coil. The magnitude of the emf is proportional to the velocity of the coil in the magnetic field.
In some applications it is desireable to determine the acceleration of the geophone housing, instead of the velocity. One known method of determining the acceleration of the geophone housing is to utilize a force-balance feedback geophone. In a force-balance feedback geophone, feedback is used to substantially limit the movement of the coil within the magnetic field. In other words, any movement of the coil within the magnetic field is counteracted by a force generated by passing an opposing current through the coil. The magnitude of the current, and accordingly, the magnitude of the voltage associated with the current, provides a measure of the acceleration of the geophone housing.
Utilizing feedback principles, however, imposes design constraints on design of the geophone. Geophone system elements generally should be within a prescribed range to ensure system stability. In addition, the feedback design must account for system damping and bandwidth requirements.
In addition, use of a force-balance feedback geophone presents operational problems. One attribute of an accelerometer, such as a force-balance feedback geophone, is the frequency response of the accelerometer. Detailed knowledge of the frequency response of the geophone allows for more accurate determination of the accelerations imparted to the geophone. Geophone elements such as the coil, however, are sensitive to environmental factors such as the temperature. This sensitivity often results in variations in frequency response.
One method of determining a frequency response of a system is to apply a step input to the system. Thus, the frequency response to a step or other known input can be determined in a force-balance feedback geophone by mechanically accelerating the geophone. This may be accomplished by placing the geophone housing in, for example, a mechanical shaker. In many geophone applications the geophone housing is placed in situations of temperature extremes and in which placement of a mechanical shaker is inconvenient or otherwise not feasible. Thus, the use of mechanical shakers is not always possible.
Conventional geophones not using force-balance feedback may be provided a step input by generating a current in the coil, thereby generating a force which results in movement of the coil within the magnetic field. In a force-balance feedback geophone, however, the feedback circuitry automatically acts to cancel any movement of the coil, and no movement of the coil will occur. Thus, although force-balance feedback geophones allow for direct measurement of accelerations, they also impose difficulties in accurately characterizing the response of the geophone to accelerations.
The present invention provides a force-balance feedback geophone and a method of characterizing a force-balance feedback geophone. In one embodiment the force-balance feedback geophone comprises a geophone housing having a coil mounted by a spring, the coil being in a magnetic field and having a first terminal and a second terminal. The force-balance feedback geophone further comprises an amplifier with a feedback circuit forming a feedback path, the amplifier having an input coupled to the first terminal. A feedback switch is in the feedback path, and the feedback switch is responsive to a control input. The feedback switch has a first state opening the feedback path and a second state closing the feedback path. The force-balance feedback geophone further comprises a bias switch coupled to the first terminal or second terminal. The bias switch is responsive to the control input and has a first state coupling the first terminal or the second terminal to a bias input and a second state not coupling the first terminal or the second terminal to the bias input.
In one embodiment, a method of the present invention comprises disconnecting a feedback element of a force-balance feedback geophone from a coil of the force-balance feedback geophone. The coil of the force-balance feedback geophone is displaced, and the feedback element of the force-balance geophone is connected to the coil of the force-balance feedback geophone. The output of the force-balance feedback geophone is then measured. In one embodiment, the feedback element is disconnected using an electrically controlled switch. In one embodiment the electrically controlled switch is a transistor.
Many of the attendant features of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description considered in connection with the accompanying drawings in which like reference numerals indicate like parts throughout.